There is known a direct-type tire inflation pressure detector according to the related art. This type of tire inflation pressure detector uses a transmitter that is provided with a sensor such as a pressure sensor and is directly attached to a vehicle mounted with tires. An antenna and a receiver are provided for the vehicle. If the transmitter transmits a detection signal from the sensor, the receiver receives the detection signal at the antenna and detects the tire inflation pressure.
The direct-type tire inflation pressure detector determines whether the data is transmitted from a local vehicle and to which wheel the transmitter is attached. For this purpose, each data transmitted from the transmitter contains ID information that discriminates between a local vehicle and a remote vehicle and identifies a wheel to which the transmitter is attached.
The receiver needs to previously register the ID information about each transmitter in association with each wheel position to be able to locate the transmitter from the ID information contained in the transmission data. If a tire is replaced, the receiver needs to re-register the positional relationship between the transmitter ID information and the wheel. For example, patent document 1 proposes the technological method of automating this registration.
Specifically, the apparatus described in patent document 1 detects that the wheel reaches a specified rotation position based on an acceleration detection signal from an acceleration sensor included in the transmitter provided for a vehicle. The vehicle also detects a rotation position of the wheel when a wireless signal is received from the transmitter. The vehicle monitors a change in a relative angle between the rotation positions to specify the wheel position. This method monitors a change in the relative angle between the wheel rotation position detected by the vehicle and the wheel rotation position detected by the wheel based on the deviation in the specified number of pieces of data. The method specifies the wheel position by determining that a variation exceeds an allowable value with reference to an initial value. In more detail, the method finds the number of gear (rotor) teeth from a wheel speed pulse output from a wheel speed sensor provided for the corresponding wheel. A rotation position is detected at a wheel based on an acceleration detection signal from an acceleration sensor. The number of gear teeth found from the wheel speed pulse of the wheel speed sensor indicates a rotation angle. The method specifies a wheel position based on a relative angle between the rotation position and the rotation angle.
The method described in patent document 1 finds the number of gear teeth at a specified cycle from the wheel speed pulse output from the wheel speed sensor. There is a difference between the timing to acquire the number of teeth and the timing to detect the rotation position based on the acceleration detection signal from the acceleration sensor at the wheel side. The difference is irregular and increases a variation in the relative angle between the rotation position and the rotation angle. As described above, the rotation position is detected at the wheel based on an acceleration detection signal from the acceleration sensor. The number of gear teeth found from the wheel speed pulse of the wheel speed sensor indicates the rotation angle. The wheel position cannot be specified in a short period of time.
The method described in patent document 1 specifies the wheel position based on whether a variation belongs to an allowable range defined by a specified allowable value with reference to an initial value. The method cannot specify the wheel position while the variation belongs to the allowable range. A certain amount of data is needed because the method specifies the wheel position based on the standard deviation. The method cannot specify the wheel position until the necessary amount of data is acquired. Accordingly, specifying the wheel position consumes the time.    Patent Document 1: JP-A-2010-122023